A conventional refueling platform, or gantry, is used for transporting nuclear fuel bundles over a nuclear boiling water reactor, for example. The platform spans the reactor and spent fuel water pool and runs on platform guide rails typically embedded in a concrete floor. The platform is used for removing spent fuel bundles from the reactor core and transporting them underwater to the spent fuel pool, as well as transporting new fuel bundles from a storage area to the reactor core.
The platform typically includes a bridge spanning the water pool which is joined to left and right drive trucks by end frames. The bridge includes a trolley which is selectively positionable along the longitudinal axis of the bridge between the end frames. The trolley includes a main hoist for selectively raising and lowering fuel bundles in the water. The bridge is selectively movable in forward and reverse transverse directions perpendicular to the bridge longitudinal axis along the guide rails.
Accordingly, a fuel bundle may be raised and lowered by the main hoist, and may be translated left and right by the trolley along the longitudinal axis of the bridge, and may also be translated in the transverse direction upon forward and reverse movement of the drive trucks supporting the bridge.
The bridge typically spans a water pool of about 45 feet (13.7 meters) and is sized for translating under water at speeds between 0 and about 50 feet per minute (15.2 meters per minute) fuel bundles weighing about 750 pounds (340 kilograms). In order to effectively translate the fuel bundle under water, the bridge must be made substantially rigid for accommodating the fuel bundle weight, and inertia and drag forces from movement of the fuel bundle under water without undesirable distortion. To accommodate the relatively high weight of the refueling platform, the installed guide rails typically have a rating of about 35 pounds (15.9 kilograms) in the original facility design.
The refueling platform also includes a drive system for translating the bridge in the forward and reverse directions along the guide rails. The drive system typically includes a single electrical motor driving a double output shaft gearbox that drives corresponding chain and sprocket final drive assemblies for driving a bridge driven wheel located in each of the left and right drive trucks.
A second design of conventional refueling platform includes a single motor driving a double output shaft gearbox that drives corresponding right angle gearbox drive assemblies in each of the left and right drive trucks. This design of platform is conventionally sized more rigidly than the above first design platform using chain and sprocket drives, and requires higher rated guide rails, for example rated at about 85 pounds (38.6 kilograms).
In both of the above conventional single motor double output refueling platform designs, skewing, or differential transverse travel of the left and right drive trucks may occur during operation. For example, as the fuel bundle load joined to the main hoist and trolley is positioned off center along the bridge longitudinal axis, a torque is generated on the bridge from the inertia and drag forces of the fuel bundle in the water as the bridge is moved transversely along the guide rails. This skewing torque temporarily slows down travel of one of the two left and right trucks relative to the other. The differential transverse travel between the drive trucks is a function of the inertia and drag of the fuel bundle, its position and trolley position from the center of the bridge, and the velocity and acceleration of the bridge in its transverse motion along the guide rails. Substantially no skewing occurs when the fuel bundle is placed at the center of the bridge, and a maximum amount of skewing occurs when the fuel bundle is placed at either the left or right end of the bridge.
The differential transverse travel between the left and right drive trucks is due in part to the structural flexibility of the bridge. It is also due in part to inherent backlash found in the transmissions joining the motor to the drive trucks. The transmissions which include chain and sprocket reduction drive assemblies, gear reduction drive assemblies, and/or drive shafts typically have backlash, or a hesitation or lag before the motor rotates sufficiently to begin rotation o the driven wheel. The differential transverse travel may also be due to any slippage of a driven wheel on the guide rail which would allow the other driven wheel to advance itself relative thereto.
In operation of the exemplary chain driven fueling platform described above, such skewing has resulted in up to about 2 feet (0.6 meters) of differential transverse travel between the left and right trucks which has occasionally caused the trucks to bind on the guide rails and, therefore, prevent further travel. To release the trucks from their binding condition, heavy duty equipment was required.
Although the second design refueling platform described above is substantially more rigid than the first design, and therefore subject to less differential transverse travel of the drive trucks, it is also more expensive than the first platform and requires more expensive higher rated guide rails.